Field of the Invention
The present invention relates to metal detectors and methods for sensing and indicating the proximity of selected target objects or features concealed within or beneath surfaces such as soil or other strata.
Discussion of the Prior Art
Metal detectors have been used by civilians and military personnel for a variety of reasons. Commonly, the civilian use of metal detectors includes the enjoyment of a hobby that, in many cases, is intended to pay for itself, meaning that individuals often purchase metal detectors for assistance in finding and recovering lost articles such as coins, jewelry and other ferrous or non-ferrous metal objects.
Commercially available metal detectors have become increasingly user-friendly and a number of commercial “all-purpose” metal detectors are being sold to users hoping to recover coins jewelry or historical relics. Many engaged in the hobby of “metal detecting” frequent beach or oceanside settings and thus use the metal detectors in damp environments, possibly including salty ocean water. Modern metal detectors often include waterproof “search coils” in a pickup array sensor mounted at the distal end of an elongated adjustable shaft. A typical metal detector may include a number of features which can be selected or adapted to a particular user's purpose, and there are different types of detectors in a variety of weights. Often, the adjustable shaft has a minimum or maximum length that permits use of the detector when the user is standing, and may be adjustable for use in some other orientation. Typically a detector includes a rechargeable system with batteries of a selected configuration.
Metal detectors usually are capable of generating an audible signal which varies depending on the proximity of the pickup array sensors to a target or object in an environment. Traditional metal detectors are usually used in conjunction with a headset or a loud speaker which provides an audible indication of the response of the pickup array sensors, and audio tones can be selected and adjusted in type, frequency and volume. Additional audible signals may include a low battery warning or other audible indicia. There is often an armrest which may include a strap and a control box to which the user attaches the headset, or headphones, for use in listening to the audible signal. Detectors often include additional controls for selecting the detector excitation signal from a number of available frequencies. Some metal detectors include interchangeable search coils which may be waterproof or submersible, while some include optional search coils which are adapted for use in selected environments or over selected surfaces.
Usually, metal detectors include a display illustrating the status of the pickup array or sensor, and may incorporate alphanumeric or graphical data indicating something about the settings of the metal detector and the response signals generated in the search coil. A number of modern metal detectors have different “search modes” and may include features designed to allow discrimination of different targets or selectable thresholds or sensitivities. Ground balancing can be fixed, manual, or automatic and target identification can be selected or programmable in one or more target ID segments. There are also metal detectors with depth indication for sensed target items or objects and some metal detectors provide spatial resolution adjustments including a “pinpoint mode”. The simplest metal detectors consist of an oscillator producing an alternating current that passes thru a coil to produce an alternating magnetic field. If an object or item including some electrically conductive material, which may be referred to as the target, is close to the coil, eddy currents will be induced in the target, thereby generating an alternating magnetic field of its own. The induced alternating magnetic field is readily sensed in another coil in the detector which acts as a magnetometer and is used to measure that magnetic field. Any change in the sensed magnetic field caused by proximity to a conductive object can be detected, and such changes can be indicated by generating a corresponding audible tone or other indicia.
Metal detectors have been used to detect mines in military applications and industrial and commercial metal detectors developed in the sixties have been used extensively for mining and other industrial applications. Similar sensors are also used in the detection of weapons such as knives and guns, especially in airport security settings. Metal detectors have also been used to detect foreign objects in food and in the construction industry to detect steel reinforcing bars in concrete and pipes and wires buried within walls and floors.
One challenge for users of metal detectors is that an object may be sensed at or just below the surface of the ground, but the information about the sensed object or target item may not allow the metal detector user to determine whether the sensed item is worth recovering. In response, metal detector manufacturers have developed an induction-balance system involving two or more coils that are electrically balanced. When metal, or other electrically conductive material, is introduced into their vicinity the coils become unbalanced. The fact that every metal has a different phase response when exposed to alternating current (e.g., ferrous v. non-ferrous) allows detectors to discriminate between metals. Metal detector designers have thus attempted to develop detectors which can reliably and selectively detect desirable metals while ignoring undesirable metals. Even with discriminators, however, it is still a challenge to avoid undesirable metals because some of them, such as tin foil and gold, have similar phase responses. Discriminators may also reduce the sensitivity of the metal detector.
Coil designers have also tried out a number of configurations and orientations. Compass Electronics produced a design including 2 coils in a D shape mounted back to back to form a circle. Another development was the invention of detectors designed to cancel out the effect of mineralization in the ground, thereby giving greater detection depth, but this feature provided poor discrimination of the sensed items or targets. Many detectors in the '70s included a switch enabling the user to switch between a “discriminate” mode and a “non-discriminate” mode, thereby allowing the user to decide whether sensitivity or discrimination was a more desirable characteristic for a given environment. The induction balance detector led to the development of a motion detector in the metal detector unit which was alleged to constantly check and balance background mineralization.
A number of companies refined their metal detector designs and their work appears to be well represented in the patented prior art. For example, U.S. Pat. No. 6,421,621, assigned to Whites Electronics, describes a metal detector target identification system using “flash phase analysis” and defining what are characterized as “phase windows”. This metal detector is intended to provide information about the target type.
The Minelab company recently received U.S. Pat. No. 7,924,012 on an electronic metal detector having a constant reactive transmit voltage applied to a transmit coil for transmitting an alternating magnetic field. The transmit electronics has at least two power sources, a first source connected to the transmit coil for a first period, and a second source which alternates with the first source to provide essentially constant excitation of the transmit coil and essentially continuous indicator output.
U.S. Pat. No. 5,786,696 is assigned to Garret Electronics and describes a metal detector for identifying target electrical characteristics, depth and size. The '696 patent describes digital signal processing methods used to sense predicted responses from coins, rings, pull-tabs, bottle caps, and ferrous soil so that an indication of the type of target can be provided to the user over an audio circuit and visually thru a display.
None of the metal detectors in the prior art provide a satisfactory solution to the problems frustrating metal detector users. In addition to unreliable discrimination between valuable and worthless items, all of the prior metal detectors are frustrated by “false positive” indications in wet conductive dirt or clay for users attempting to locate dangerous metal objects such as nails or valuable objects that may be on or near the surface of the soil. The applicant for the present invention has personally experienced this problem when using a very expensive commercial metal detector. Efforts to find nails in moist clay provided repeated positive indications. When the soil which was indicated to contain the detected metal object was removed from the site, that soil was again swept and a (false) indication was again observed. When the soil was broken down into smaller and smaller clumps, the applicant was frustrated to learn that there was no nail or other ferrous object and the false positive indication was simply the result of moisture in the soil.
Another problem found in the prior art is a lack of continuity in the detection signal when the detector is not moving; the audio signal indicating a target is only present when the detector is in motion, and fades away when the detector is stopped over or near a target. This makes it difficult to locate a target object with precision.
What is needed, then, is a metal object or feature detection apparatus and method which will overcome the shortcomings of the prior art.